Non-KREEP origin for Chang’e-5 basalts in the Procellarum KREEP Terrane

Mare volcanics on the Moon are the key record of thermo-chemical evolution throughout most of lunar history1–3. Young mare basalts—mainly distributed in a region rich in potassium, rare-earth elements and phosphorus (KREEP) in Oceanus Procellarum, called the Procellarum KREEP Terrane (PKT)4—were thought to be formed from KREEP-rich sources at depth5–7. However, this hypothesis has not been tested with young basalts from the PKT. Here we present a petrological and geochemical study of the basalt clasts from the PKT returned by the Chang’e-5 mission8. These two-billion-year-old basalts are the youngest lunar samples reported so far9. Bulk rock compositions have moderate titanium and high iron contents with KREEP-like rare-earth-element and high thorium concentrations. However, strontium–neodymium isotopes indicate that these basalts were derived from a non-KREEP mantle source. To produce the high abundances of rare-earth elements and thorium, low-degree partial melting and extensive fractional crystallization are required. Our results indicate that the KREEP association may not be a prerequisite for young mare volcanism. Absolving the need to invoke heat-producing elements in their source implies a more sustained cooling history of the lunar interior to generate the Moon’s youngest melts.

Article lava flow. First, the initial plagioclase 87 Sr/ 86 Sr ratios of 11 clasts and the merrillite ε Nd (t) of three clasts are nearly identical (Fig. 3). Second, most of the pyroxene grains in these clasts fall along the 1:2 line on a titanium/aluminium (Ti/Al) diagram (Extended Data Fig. 3b), reflecting the near-simultaneous crystallization of augite and plagioclase during one cooling event 11 . Lastly, both the pyroxene and plagioclase grains measured in different clasts show similar REE patterns (Extended Data Fig. 4), like those in the Apollo 12 and Apollo 15 basalts with different cooling rates 12 . Therefore, the range in textures of the Chang'e-5 basalt clasts is probably due to different cooling rates 13 within different parts of the lava flow. The appreciable variation of mineralogical abundances and inferred whole-rock compositions are probably due to the small sample sizes (smaller than 3 mm) of the basalt clasts.
For comparison with previous results from the Apollo and Luna samples and remote sensing, the bulk-rock major and trace-element compositions of the Chang'e-5 basalts are estimated (Extended Data Tables 5,6). The Chang'e-5 basalt clasts can be classified as low-Ti/high-Al/low-potassium (K) type 14 . However, these basalts have higher iron (FeO; 22.2 wt%), titanium (TiO 2 ; 5.7 wt%) and aluminium (Al 2 O 3 ; 11.6 wt%) contents and a lower Mg# (32.1) relative to the Apollo and Luna low-Ti basalts (Fig. 2b, Extended Data Fig. 5). In addition, the Chang'e-5 clasts are highly LREE enriched (50 times heavy REE (HREE) and 150 times LREE enrichment relative to carbonaceous chondrites; Fig. 4a), and show high thorium (Th) contents (approximately 4.5 ppm; Extended Data Table 6). The REE patterns are different from those of the Apollo low-Ti basalts, but parallel to those of the KREEP basalts (Fig. 4a). In addition, the FeO, TiO 2 and Th contents of the Chang'e-5 basalt agree well with the data obtained by the Lunar Prospector Gamma-Ray Spectrometer [15][16][17] (FeO about 22.4 wt%; TiO 2 about 4.5 wt%; Th about 5.8 ppm). This agreement suggests that the Chang'e-5 basalt clasts are representative of the mare basalt unit of the landing site (designated as unit P58; ref. 18 ).
The elevated incompatible trace element (ITE) concentrations and LREE enrichment in the Chang'e-5 basalt clasts are typical characteristics  for the KREEP-rich materials. However, the isotopic compositions of the Chang'e-5 basalt are not consistent with the origin of KREEP-rich materials (Fig. 3). Even a small contribution (less than 0.5%; Extended Data Fig. 6) from KREEP materials would result in high 87 Rb/ 86 Sr ratios (greater than 0.19) and low 147 Sm/ 144 Nd ratios (less than 0.173) (Fig. 3), which would shift the Sr-Nd isotopes of the Chang'e-5 basalts considerably. The low initial 87 Sr/ 86 Sr and high ε Nd (t) observed in the Chang'e-5 basalts are similar to those of the Apollo 12 low-Ti basalts. This similarity indicates that both the Chang'e-5 basalts and the Apollo 12 low-Ti basalts may originate from a depleted non-KREEP source, which may have crystallized from the early lunar magma ocean cumulates dominated by olivine and pyroxene 19 . The REE and ITE enrichment most likely formed through magmatic processes, such as partial melting and fractional crystallization. Similar characteristics observed in lunar basalt meteorites (for example, NWA 032, NWA 4734 and LAP 02205) were considered to originate from low-degree partial melting of a depleted source 20,21 . We suggest that a cumulate composition with a crystallized percent solid at 86% and the addition of 2% trapped instantaneous residual liquid in the model of ref. 22 can produce a source with 147 Sm/ 144 Nd ≈ 0.222-0.227 and 87 Rb/ 86 Sr ≈ 0.009-0.022 (Fig. 2). Partial melting of such a mantle source alone cannot simultaneously reproduce the LREE and HREE contents even with an unrealistic, low degree of melting (less than 0.3%) (Extended Data Fig. 7). Thus, fractional crystallization must have occurred to elevate the ITE and LREE abundances before the eruption of the Chang'e-5 basalts. Even the parental melt estimated by the clinopyroxene core with the highest Mg# (sample 406-004, 005) (Extended Data  The blue areas denote the melts produced after 2-3% partial melting of the mantle source (86 PCS + 2% TIRL 22 ; PCS, per cent crystallized solid; TIRL, trapped instantaneous residual liquid) and those followed by 43-78% fractional crystallization. This source composition is calculated based on the source region 87 Rb/ 86 Sr and 147 Sm/ 144 Nd ratios (Fig. 3). Mineral modes in the source are assumed to be 48% olivine, 23% orthopyroxene, 23% pigeonite, 3% augite and 3% plagioclase. Mineral assemblages of 43-78% crystallization are: 5-10% olivine, 25-59% augite, 2-3% pigeonite and 6-11% plagioclase. Normalization values are from ref. 43 . The Chang'e-5 parental melt estimated by the clinopyroxene core with the highest Mg# (sample 406-004, 005) (Extended Data Table 6) requires extensive (43-78%) fractional crystallization after low-degree (2-3%) melting of the mantle source. To match the Chang'e-5 bulk composition, up to 78-88% fractional crystallization is needed. The model parameters are listed in Supplementary Table 3, and details of the batch melting and fractional crystallization model are provided in the Methods.

Article
requires extensive (43-78%) fractional crystallization after low-degree (2-3%) melting of the mantle source ( Fig. 4). This scenario is consistent with the low-Mg# (32.1), high-FeO (22.2 wt%) and high-TiO 2 (5.7 wt%) signatures (Table 1), and compositional zonings in olivine and pyroxene of the samples (Extended Data Fig. 2). Therefore, the Chang'e-5 basalts are highly evolved magmatic products, which implies the presence of a huge magma chamber beneath the PKT at that time.
The youngest mare basalt units in the PKT (for example, P56, P58, P59 and P60; ref. 18 ) all show moderate-TiO 2 (about 3.8-5.7 wt%) and high-Th (4.9-7.3 ppm) contents 15,23 . The high-Th materials excavated by impact craters indicate a KREEP layer beneath the PKT 24 . Melting for the prolonged volcanism in the PKT was thought to be driven by the high abundances of radiogenic heat-producing elements from the KREEP-rich materials [5][6][7] . However, we find that the elevated Th and other incompatible elements of the Chang'e-5 basalts from the mare unit P58 were not involved with KREEP-rich materials, but rather attributable to the highly evolved basalt produced by low-degree partial melting and extensive fractional crystallization. We deem it unlikely that the KREEP-rich materials provided the heat for partial melting without being involved in the melt itself 7 . Therefore, the presence of a speculative thick KREEP layer at the base of the crust to generate the young volcanism in the PKT region is not required.
The highly evolved origin of the 2 billion-year-old (Ga) Chang'e-5 basalts implies that the lunar interior was substantially cooler at that time than at around 3.5 Ga when the variety of more primitive basalts sampled by Apollo were formed. In spite of this considerable secular cooling, there must also have been some mechanisms to keep the melt zones in the lunar mantle from solidifying until after 2 Ga (ref. 2 ). One possible mechanism is that a thick insulating outer layer of the Moon called megaregolith 2,25 served as a thermal lid, resulting in a sufficiently slow cooling rate. In addition, lunar cooling can thicken the lithosphere and thus inhibit surface eruption 26 . Therefore, the thinnest crust of the PKT region (typically less than 30 km; ref. 27 ) could be a critical factor that facilitated the eruption of young basalts 26 . Lastly, evidence for the lunar magnetic field persisting until sometime after around 1.92 Ga (ref. 28 ) is consistent with sources of lunar interior heat flow such as the latent heat of crystallization of the solid inner core 29 being sustained until the age of the Chang'e-5 basalts. Any new model for the thermal evolution of the Moon needs to fit the observation of a non-KREEP origin for the youngest basalts of the PKT.

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Sample preparation
The Chang'e-5 samples studied in this work are two one-inch epoxy mounts (CE5C0000YJYX041GP, 2 mg; CE5C0000YJYX042GP, 44.6 mg) and two soil samples (CE5C0100YJFM00103, about 1,000 mg; CE5C0400YJFM00406, about 2,000 mg) allocated by the China National Space Administration. All these samples were scooped from the lunar surface. The two one-inch epoxy mounts, each containing two basalt clasts, were already polished before allocation. For soil samples, clasts were picked up by a sieve (aperture 600 μm) and hand-picked. Then, the clasts were embedded in epoxy mounts and polished using the grinder.

Scanning electron microscopy analysis and energy dispersive spectrometry mapping
The petrography was carried out on a Zeiss Gemini 450 field emission scanning electron microscope at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The acceleration voltage was 15.0 KV and the probe current was 2.0 nA. In addition, a Thermo Scientific Apreo scanning electron microscope equipped with an energy dispersive spectrometer was used to identify the phosphorus/zirconium-bearing minerals and to calculate the modal abundance of each mineral based on the elemental mapping.

Electron microprobe analysis of minerals
The major element concentrations of pyroxene, plagioclase, olivine, ilmenite, spinel, quartz, sulfide and phosphates in each sample were analysed by a JEOL JXA8100 electron probe at the IGGCAS. The conditions of the electron microprobe analysis were: acceleration voltage of 15 kV, probe current of 20 nA, focused beam and peak counting time of 10 s. The calibration of the elemental data was done using a series of natural minerals and synthetic materials. On the basis of the analysis of internal laboratory standards, the precision for major (more than 1.0 wt%) and minor (less than 1.0 wt%) elements are better than 1.5% and 5.0%, respectively.

In situ trace-element analysis
The trace-element abundances of pyroxene and plagioclase in basalt fragments were determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) employing an Element XR HR-ICP-MS instrument coupled to a 193-nm argon-fluoride excimer laser system (Geolas HD) at the IGGCAS. The approach is similar to that outlined in ref. 44 with isotopes measured using a peak-hopping mode. The laser diameter is about 32 μm with the repetition rate of 3 Hz. The laser energy density is approximately 3.0 J cm −2 . The Element XR is equipped with a high-capacity interface pump (OnTool Booster 150) in combination with Jet sample and normal H-skimmer cones to achieve a detection efficiency in the range of 1.5% (based on U in a single spot ablation of NIST SRM 612). Helium was employed as the ablation gas to improve the transporting efficiency of ablated aerosols. NIST SRM 610 (ref. 45 ) reference glass was used for external calibration. ARM-1 (ref. 46 ), BCR-2G (ref. 47 ) and BIR-1G (ref. 47 ) glasses were used for quality control monitoring. The bulk normalization as 100 wt% strategy was used for data reduction, which is accomplished using the Iolite software package with an in-house-built data reduction scheme code 48 . For most trace elements (more than 0.05 ppm), the accuracy is better than ±15% with analytical precision (1 relative standard deviation) of ±10%. The spots analysed for trace elements are shown in Supplementary Figs. 1-3.

In situ Sr-Nd isotopic analysis
In situ Sr-Nd isotopic measurements by LA-multiple collector (MC)-ICP-MS followed the method of refs. [49][50][51] , hence only a brief description is given below. All analyses were conducted at the IGGCAS.
A Neptune Plus MC-ICP-MS coupled to an Analyte G2 193-nm argon-fluoride excimer laser ablation system was used to determine the Sr isotopic ratios of the plagioclase. A spot size of 85 μm was employed with a repetition rate of 6 Hz and an energy density of approximately 7 J cm −2 . The Sr isotopic data were acquired by static multi-collection in low-resolution mode using nine Faraday collectors. Before laser analysis, the Neptune Plus MC-ICP-MS was tuned using NBS 987 standard solution to obtain maximum sensitivity. A typical data acquisition cycle consisted of a 30 s of measurement of the krypton gas blank with the laser switched off, followed by 60 s of measurement with the laser ablating. Data reduction was conducted offline and the potential isobaric interferences were accounted for in the following order: Kr + , Yb 2+ , Er 2+ and Rb + . Finally, the 87 Sr/ 86 Sr ratios were calculated and normalized from the interference-corrected 86 Sr/ 88 Sr ratio using an exponential law. The whole data-reduction procedure was performed using an in-house Excel VBA (Visual Basic for Applications) macro program. The JH56 in-house plagioclase reference material was measured before and after unknown samples for external calibration 49 , which are shown in Extended Data Table 3.
The same LA-MC-ICP-MS system as for the in situ Sr isotopic analysis was used to determine Nd isotopic compositions of the merrillite. Before laser analyses, the Neptune Plus MC-ICP-MS was tuned and optimized for maximum sensitivity using JNdi-1 standard solution.
A laser spot size of 20 μm was employed with a repetition rate of 3 Hz and an energy density of approximately 6 J cm −2 . Each spot analysis consisted of approximately 60 s of data acquisition with the laser fire on. The SDG in-house apatite reference material was measured before and after the merrillite samples.
To obtain accurate 147 Sm/ 144 Nd and 143 Nd/ 144 Nd data by LA-MC-ICP-MS, care must be taken to adequately correct for the contribution of the isobaric interference of 144 Sm on the 144 Nd signal. The (samarium) Sm interference correction is complicated by the fact that the 146 Nd/ 144 Nd ratio, which is conventionally used to normalize the other Nd isotope ratios, is also affected by Sm interference. As a result, the mass bias correction of 144 Sm interference on 144 Nd cannot be applied directly from the measured 146 Nd/ 144 Nd ratio. In this work, we adopted the recently revised Sm isotopic abundances ( 147 Sm/ 149 Sm = 1.08680 and 144 Sm/ 149 Sm = 0.22332) 50 . First, we used the measured 147 Sm/ 149 Sm ratio to calculate the mass bias factor of Sm and the measured 147 Sm intensity by employing the natural 147 Sm/ 144 Sm ratio of 4.866559 to estimate the Sm interference on mass 144. Then the interference-corrected 146 Nd/ 144 Nd ratio can be used to calculate the mass bias factor of Nd. Finally, the 143 Nd/ 144 Nd and 145 Nd/ 144 Nd ratios were normalized using the exponential law. The 147 Sm/ 144 Nd ratio of unknown samples can also be calculated using the exponential law after correcting for isobaric interference of 144 Sm on 144 Nd as described above. The 147 Sm/ 144 Nd ratio was then externally further calibrated against the 147 Sm/ 144 Nd ratio of the SDG reference material during the analytical sessions 50 . The raw data were exported offline and the whole data-reduction procedure was performed using an in-house Excel VBA macro program. The LREE glass analysed in this session gave a mean 143 Nd/ 144 Nd ratio of 0.512100 ± 0.000048 (2 s.d., n = 8), which is consistent with the recommended value 52 . The data are shown in Extended Data Table 4. The spots analysed for Sr-Nd isotopes are shown in Supplementary Figs. 1-3.
The subophitic clasts show various grain sizes (30-300 μm) and consist mainly of plagioclase, clinopyroxene, olivine and ilmenite, with minor troilite and cristobalite (Extended Data Fig. 1b). Both clinopyroxene and olivine have compositional zoning, with Mg-rich cores and iron-rich rims (Extended Data Fig. 2, Supplementary Table 1). Plagioclase has a euhedral-to-subhedral shape with an anorthite-rich composition (An 75. 7-90.3 ). Small amounts of Fe-rich olivine (Fo < 10) associated with cristobalite and apatite occur as mesostasis phases representing the late-stage crystallization products.
Except for the above four types of clast, very small amounts of coarse-grained (larger than100 μm) fragments of basalt clasts can be found. However, their texture cannot be identified well owing to the limited (about two to five) grains in a single clast. These fragments typically contain coarse-grained clinopyroxene, plagioclase and olivine, with minor fine-grained (mostly smaller than 30 μm) silica and phosphate minerals.

Estimation of bulk composition
It is assumed that the analysed area proportions are equal to volume proportions and the volume proportions are then converted to mass proportions based the mineral densities reported in previous studies [53][54][55] . The bulk composition is then calculated by their mass proportions. The oxide concentrations for the bulk composition are normalized to 100%. The average compositions of all samples are calculated based on the contribution for each sample multiplied by its weight, assuming that the weight of each sample is in proportion to its surface area (Supplementary Table 1 an extremely high abundance of plagioclase (72.8%). Clast 406-002, 007 has a very high abundances of fayalitic olivine (22.6%) and silica (7.8%). Clast 406-002, 002 has an extremely high abundance of ilmenite (19.1%). These outlier clasts are excluded for the estimation of the bulk composition of the Chang'e-5 basalt (Extended Data Table 5).
The bulk trace elements of the Chang'e-5 basalt were estimated based on the average of all measured pyroxene grains and the partition coefficients (Extended Data Table 6). This estimation assumes that the Chang'e-5 basalt crystallized in a closed system and can be represented by the equilibrium melt of pyroxene. This method may yield large uncertainties owing to the significant variations of the trace-element contents of pyroxene. Therefore, the parental melt estimated by the clinopyroxene core with the highest Mg# (sample 406-004, 005; Extended Data Table 6) is used for the REE modelling of partial melting and fractional crystallization (Fig. 4).

Batch melting model
In this work, we use the batch melting model to calculate the REE concentrations in the parental melts, assuming that each mineral phase melts in proportion to its modal abundance in the source. The batch melting is calculated using the following equation: C L /C 0 = 1/[D 0 + F(1 − D 0 )], where C L is the weight concentration of a trace element in the melt, C 0 is the weight concentration of a trace element in the original cumulate source, F is the weight fraction of melt produced and D 0 is the bulk distribution coefficient of the original solid material.
The bulk distribution coefficient is calculated by multiplying each mineral partition coefficient by the fraction of that mineral in the source. The REE partition coefficients for olivine 56 , orthopyroxene 57 , augite 57 , pigeonite 58 and plagioclase 59 , and the Th partition coefficient for augite 60 are shown in Supplementary Table 3. Given that the Chang'e-5 basalts have a similar source to the Apollo 12 basalts, the modal mineralogical assemblage calculated for Apollo 12 19 are also adopted here but with a little modification owing to the geochemical differences between Chang'e-5 and Apollo 12 mare basalts. The source materials are compiled in Extended Data Table 6.
Using the bulk distribution coefficients (D 0 ) and solid cumulate (C 0 ), the weight concentration of REE in the melt (C L ) is calculated for increasing melt fractions (F).

Fractional crystallization model
The trace-element concentrations in the remaining melt induced by fractional crystallization are calculated using the Rayleigh fractionation equation: C L /C 0 = (1 − F) D−1 , where D is bulk distribution coefficient (the same as described in batch melting model), F is the mass fraction of crystals crystallized from the melt, C 0 is the concentration of an element in the initial melt and C L is the concentration in the final melt. The initial melts during the calculation are assumed to be derived from 2% and 3% batch melting of the mantle source, and the results are shown in Fig. 4.

Data availability
All data generated or analysed during this study are available in Earth-Chem Library at https://doi.org/10.26022/IEDA/112076. Source data are provided with this paper.